Mobile device with on-screen optical navigation

ABSTRACT

A mobile computing device, including a main body, a processor and associated memory housed within the main body, a display screen housed within the main body and responsive to signals from the processor, an optical sensor fixed to the main body for capturing successive images and providing image signals representative of the captured images to the processor, and a navigation module associated with the processor for determining, based on the image signals, a relative movement between the main body and a reference surface and moving a pointer on the display screen based on the determined relative movement.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 10/911,583, filed Aug. 5, 2004, and which claims priority toUnited Kingdom patent application serial no. 0318358.9 filed Aug. 5,2003, both contents of which are hereby incorporated by reference.

FIELD OF THE APPLICATION

The present application relates generally to the field of mobile deviceshaving display screens and, in particular, to mobile devices havingoptical sensors.

BACKGROUND OF THE INVENTION

Mobile devices, such as personal digital assistants and messagingenabled communications devices are rapidly growing in popularity. Morefeatures are being incorporated into mobile devices. For example, thereare now messaging enabled mobile phones that have display screens andbuilt-in-cameras. Such phones allow images taken by the camera to bedisplayed on screen, stored on the phone and wirelessly transmitted asdigital photos. The use of an integrated camera as an input interfacefor a mobile communications device has been proposed, whereby imageinformation from the camera is converted to text data which can be usedas a phone number, Internet address, or mail text to support calling,Internet connection or mail passing.

Due to the compact size and portability of handheld mobile devices, therange of input interfaces for on-screen navigation is typically morelimited for such compact devices than for larger devices such as laptopand desktop computers. Rather than using optical or roller ball mice,touch pads, or joysticks to move an on-screen pointer or cursor,navigation control on handheld devices typically relies on a directionalkeypad, a stylus, and/or other input devices such as a thumbwheel.

Thus, there is a need for an alternative input interface for on-screennavigation in mobile devices.

SUMMARY OF THE INVENTION

According to the present invention, an on-board optical sensor on amobile device is used to detect relative movement of the device and areference surface, and move an on-screen pointer accordingly. In someembodiments, the optical sensor may also be used as in various modes asa camera and a scanner.

According to example aspects of the invention, there is provided amobile device, including a main body, a processor and associated memoryhoused within the main body, a display screen housed within the mainbody and responsive to signals from the processor, an optical sensorfixed to the main body for capturing successive images and providingimage signals representative of the captured images to the processor,and a navigation module associated with the processor for determining,based on the image signals, a relative movement between the main bodyand a reference surface and moving a pointer on the display screen basedon the determined relative movement.

According to other aspects of the invention, there is provided anon-screen navigation method for a mobile device having a display screen.The method includes (a) fixing an optical sensor for movement with thedisplay screen; (b) displaying an on-screen pointer on the displayscreen; (c) capturing successive images of a reference surface throughthe optical sensor; (d) comparing successive captured images todetermine a relative movement between the optical sensor and thereference surface; and e) moving the on-screen pointer on the displayscreen based on the determined movement.

According to yet a further aspect of the present application, there isdisclosed a mobile device. The mobile device includes a main body havinga front face and opposing back face; a processor and associated memoryhoused within the main body; a display screen mounted in the front face;an optical sensor mounted in the back face for capturing successiveimages and providing image signals representative of the captured imagesto the processor; and a navigation module executable by the processorfor converting the image signals into lower resolution image signals anddetermining, based on the lower resolution image signals, a relativemovement between the main body and a reference surface and moving apointer on the display screen based on the determined relative movement.

Accordingly to yet another aspect of the present application, there isdisclosed a on-screen navigation method for a mobile computing devicehaving front face with a display screen and having a back face. Themethod includes displaying an on-screen pointer on the display screen;capturing successive images of a reference surface using an opticalsensor mounted in the back face; converting the successive images intolower resolution images; comparing successive lower resolution images todetermine a relative movement between the optical sensor and thereference surface; and moving the on-screen pointer on the displayscreen based on the determined movement.

According to another aspect of the present application, there isdisclosed a mobile device. The device includes display means fordisplaying a pointer; camera means for capturing successive images andproviding image signals representative of the captured images;navigation means for receiving the image signals and determining, basedon the image signals, a relative movement between the main body and areference surface and moving a pointer on the display means based on thedetermined relative movement; and a casing having a front face and aback face, and wherein the display means is mounted in the front faceand the camera means is mounted in the back face.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the attached Figures, wherein:

FIG. 1 is a block diagram showing a mobile device to which the presentinvention may be applied;

FIG. 2 is a front view of the mobile device of FIG. 1;

FIG. 3 is a side view of the mobile device;

FIG. 4 is a back view of the mobile device;

FIG. 5 is a partial section view of the mobile device, taken along linesV-V of FIG. 4;

FIG. 6 is a block diagram of operating modules associated with operatingmodes of a camera of the mobile device according to embodiments of thepresent invention;

FIG. 7 is a block diagram showing an optical navigation processaccording to embodiments of the invention; and

FIG. 8 is a front view of a mobile device according to another exampleembodiment of the invention.

Like reference numerals are used throughout the Figures to denotesimilar elements and features.

DETAILED DESCRIPTION

Referring now to the drawings, FIG. 1 is a block diagram of a mobiledevice 10 to which the present invention is applied in an exampleembodiment. In the example embodiment, the mobile device 10 is ahandheld two-way mobile communication device having at least data andpossibly also voice communication capabilities. In an exampleembodiment, the device 10 has the capability to communicate with othercomputer systems on the Internet. Depending on the functionalityprovided by the device 10, in various embodiments the device 10 may be adata communication device, a multiple-mode communication deviceconfigured for both data and voice communication, a mobile telephone, aPDA (personal digital assistant) enabled for wireless communication, ora computer system with a wireless modem, among other things. In variousembodiments, the present invention may also be applied to handheldcomputing devices, such as PDAs and digital cameras that are not enabledfor communications.

The device 10 includes a communication subsystem 11, including areceiver 12, a transmitter 14, and associated components such as one ormore, preferably embedded or internal, antenna elements 16 and 18, and aprocessing module such as a digital signal processor (DSP) 20. In someembodiments, the communication subsystem 11 includes local oscillator(s)(LO) 13, and in some embodiments the communication subsystem 11 andmicroprocessor 38 share an oscillator. As will be apparent to thoseskilled in the field of communications, the particular design of thecommunication subsystem 11 will be dependent upon the communicationnetwork in which the device is intended to operate.

Signals received by the antenna 16 through a wireless communicationnetwork 50 are input to the receiver 12, which may perform such commonreceiver functions as signal amplification, frequency down conversion,filtering, channel selection and the like, and in some embodiments,analog to digital conversion. In a similar manner, signals to betransmitted are processed, including modulation and encoding forexample, by the DSP 20 and input to the transmitter 14 for digital toanalog conversion, frequency up conversion, filtering, amplification andtransmission over the network 50 via the antenna 18.

The device 10 includes a microprocessor 38 that controls the overalloperation of the device 10. The microprocessor 38 interacts withcommunications subsystem 11 and also interacts with further devicesubsystems such as the display 22, flash memory 24, random access memory(RAM) 26, auxiliary input/output (I/O) subsystems 28, serial port 30,keyboard or keypad 32, speaker 34, microphone 36, a short-rangecommunications subsystem 40, and any other device subsystems generallydesignated as 42. The device 10 of the present system includes anintegral camera 44 and backlight 46 that interact with microprocessor38.

Some of the subsystems shown in FIG. 1 perform communication-relatedfunctions, whereas other subsystems may provide “resident” or on-devicefunctions. Notably, some subsystems, such as keyboard 32 and display 22for example, may be used for both communication-related functions, suchas entering a text message for transmission over a communicationnetwork, and device-resident functions such as a calculator or tasklist.

Operating system software 54 and various software applications 58 usedby the microprocessor 38 are, in one example embodiment, stored in apersistent store such as flash memory 24 or similar storage element.Those skilled in the art will appreciate that the operating system 54,specific device applications 58, or parts thereof, may be temporarilyloaded into a volatile store such as RAM 26. It is contemplated thatreceived communication signals may also be stored to RAM 26.

The microprocessor 38, in addition to its operating system functions,preferably enables execution of software applications 58 on the device10. A predetermined set of software applications 58 which control basicdevice operations, including at least data and voice communicationapplications for example, will normally be installed on the device 10during manufacture. Further applications may also be loaded onto thedevice 10 through the network 50, an auxiliary I/O subsystem 28, serialport 30, short-range communications subsystem 40 or any other suitablesubsystem 42, and installed by a user in the RAM 26 or a non-volatilestore for execution by the microprocessor 38. Such flexibility inapplication installation increases the functionality of the device 10and may provide enhanced on-device functions, communication-relatedfunctions, or both. For example, secure communication applications mayenable electronic commerce functions and other such financialtransactions to be performed using the device 10.

In a data communication mode, a received signal such as a text messageor web page download will be processed by the communication subsystem 11and input to the microprocessor 38, which will preferably furtherprocess the received signal for output to the display 22, oralternatively to an auxiliary I/O device 28. A user of device 10 mayalso compose data items such as email messages for example, using thekeyboard 32 in conjunction with the display 22 and possibly an auxiliaryI/O device 28. Such composed items may then be transmitted over acommunication network through the communication subsystem 11.

The serial port 30 in FIG. 1 would normally be implemented in a personaldigital assistant (PDA)-type communication device for whichsynchronization with a user's desktop computer (not shown) may bedesirable, but is an optional device component. Such a port 30 wouldenable a user to set preferences through an external device or softwareapplication and would extend the capabilities of the device 10 byproviding for information or software downloads to the device 10 otherthan through the network 50.

A short-range communications subsystem 40 is a further component thatmay provide for communication between the device 10 and differentsystems or devices, which need not necessarily be similar devices. Forexample, the subsystem 40 may include an infrared device and associatedcircuits and components or a Bluetooth™ communication module to providefor communication with similarly enabled systems and devices. The device10 may be a handheld device.

Wireless mobile network 50 is, in an example embodiment, a wirelesspacket data network, (e.g. Mobitex™ or DataTAC™), which provides radiocoverage to mobile devices 10, although it could be any other types ofwireless networks.

With reference to FIGS. 2-4, in an example embodiment, the componentsand subsystems of mobile device 10 are housed within a hard plastic mainbody case 70 that is configured to be held with one or two hands whilethe device 10 is in use. The case 70 may include a hook (not shown) sothat it can be secured to a user's belt or pant's top, or it may be usedin conjunction with a soft case (not shown) that can be mounted to theuser's belt or pant's top and into which the mobile device 10 can beinserted for carrying. Mobile device 10 will typically be small enoughto fit inside a standard purse or suit jacket pocket. The screen 22 isvisible from the front of the device 10, as is keypad or keyboard 32.The keyboard 32 includes buttons or keys 90, 92 positioned to beactuated by the thumbs or fingers of the user. In the illustratedembodiment of FIG. 2, the keyboard 32 has relatively few keys, howeverin some embodiments, the keyboard 32 includes 26 or more alphanumericand control keys. As seen in FIG. 4, the case 70 includes asubstantially planar back wall 72, which has an opening 74 providedtherethrough. A lens 76 covers the opening 74, behind which camera 44and backlight 46 are located.

As seen in FIG. 5, in an example embodiment, the camera 44 and backlight46 are secured to the back of a printed circuit board 94 that is mountedwithin the main body case 70. In an example embodiment, the printedcircuit board 94 also supports at least some of the other hardwareelectronic components of the device 10. The camera 44 includes anoptical sensor 78 that faces lens 76 for receiving reflected light 100therethrough. The backlight 46 is positioned to shine light 98 throughthe lens 76 onto a reference surface 96 from which it is reflected backto optical sensor 78. Although not illustrated in the drawings, as willbe apparent to those persons skilled in the art, the camera 44, lens 76and backlight 46 may be adapted to the main body 70 by a variety ofother means without narrowing the scope of the invention. In a firstembodiment, the camera is fixed to the main body. In a secondembodiment, the camera 44, lens 76 and backlight 46 may be housed in asecondary housing (not shown) that is pivotally mounted to the body 70of the device 10. In yet another embodiment, the secondary housing maybe removably attached to the body 70 of the device 10.

Referring now to FIGS. 1 and 2, at least some of the specializedsoftware applications 58 that are loaded onto the device 10 will,together with operating system 54, implement graphical user interfacesthat display text 82 and/or graphics 80 on the screen 22, along with apointer 84 or other on-screen indicator that a user can selectivelyposition on screen 22 to navigate among displayed selectable text and/orgraphic images that represent data or commands. According to embodimentsof the present invention, camera 44 functions, in various modes, as auser input device for controlling the movement of on-screen pointer 84,as a camera for capturing still photo or video images, and as a scanner,among other things. The device 10 includes as one of the specializedsoftware applications an optical interface engine 60 for processingsignals received by the microprocessor 38 from the camera 44 in themanner described below. All or parts of the optical interface engine 60could, in various embodiments, be integrated into the operating system54 and/or other specialized applications 58. In some embodiments some ofthe optical interface engine functions could be implemented inappropriately configured hardware that may be located within the mainbody 70 or in the same housing as the camera 44.

FIG. 6 represents various modules of the optical interface engine 60according to example embodiments of the invention, which are associatedwith operating modes of the camera 44 when it is active. Moreparticularly, navigation module 112, digital camera module 114, andscanner module 116 are associated with an optical navigation mode, adigital camera mode, and a scanner mode, respectively. In variousembodiments, the camera 44 can be configured to have fewer or greaterthan three operating modes. In optical navigation mode, the receivedimages from the camera 44 are used to control on screen navigation, aswill be explained in greater detail below.

In digital camera mode, the camera 44 acts as a conventional digitalcamera, capturing colour still photo images or digital video images,which, among other things, can be stored as image files in a memory ofthe device 10, viewed on the screen 22, and sent as image files over thenetwork 50 to a destination address.

In scanner mode, the camera 44 is used to recognize images representingalphanumeric data and convert the captured images into digitalalphanumeric data. For example, in one embodiment, scanner mode has twosub-modes, namely a barcode scanning mode and an optical characterrecognition mode. In barcode scanning mode, the camera 44 is used toread barcode information that is then converted by device 10 to anumeric or alphanumeric value that can, among other things, be stored inmemory of the device 10, displayed on display 22, and/or transmittedover the network 50 to a destination address. In optical characterrecognition mode, recognized alphanumeric characters in scanned imagesare, among other things, stored in memory of the device 10, displayed ondisplay 22, and/or transmitted over the network 50 to a destinationaddress. Optical character recognition mode can be used to scan contactinformation from business cards to an electronic address book, forexample.

A select mode module 110 implements a select mode process for selectingamong the camera operating modes. In one embodiment, during select modeprocess, the user is presented with an on-screen list of the variousmodes, from which the user can select a desired choice using keyboard32. In other embodiments, the select mode process is configured toautomatically choose between at least two camera modes based on theimages that the camera is currently capturing. For example, when thecaptured images indicates that the camera 44 is within a thresholddistance of a surface 96, navigation mode is selected, but when thecamera 44 is further than the threshold distance from a surface 96, thedevice 10 automatically switches into a digital camera mode.

In one example embodiment, to facilitate its use in multiple modes, theoptical sensor 78 is a charge coupled device (CCD) having a relativelyhigh resolution and being color sensitive. By way of non-limitingexample, the sensor 78 could have a resolution of at least 100,000pixels, although lower resolution sensors are used in some embodiments.The camera 44 is capable of capturing successive frames of image at apredetermined frame per second capture rate.

With reference to FIG. 5, the operation of camera 44 in navigation modeaccording to embodiments of the invention will now be described ingreater detail. In an example embodiment, when the device 10 is innavigation mode, the user holds the device 10 in one or both hands sothat the back 72 of the device 10 is relatively close to a surface 96.In one embodiment, the device 10 can be held in one hand, with the otherhand being used as the reference surface 96. As the device 10 is moved,the navigation module 112 is configured to track, through camera 44, themovement of the device 10 relative to surface 96 and based on thetracked movement move the on-screen pointer 84. For example, withreference to FIG. 2 and the X-Y reference axis 86, movement of thedevice 10 relative to the X axis a set distance results in movement ofthe on-screen pointer 84 in the same direction by a scaled distance.Similarly, movement of the device 10 relative to the Y axis a setdistance results on a movement of the on-screen pointer 84 in the samedirection by a scaled distance. When the on-screen pointer 84 ispositioned at text or graphics (such as an icon) that the user desiresto select, the user presses a control key such as key 90, for example toindicate a selection. In some embodiments, the movement of on-screenpointer is in the opposite direction of the actual movement of thedevice 10—for example, movement of the device 10 in the negative Xdirection results in positive X direction of the on-screen pointer, andso on.

In an example embodiment, in navigation mode, the backlight 46 isactivated to provide incident lighting 98 onto surface 96 that isreflected to camera sensor 78. The backlight 46 can be a light emittingdiode (LED) or other lighting device, and be operated in a pulse mode toconserve battery power. In some embodiments, the navigation module 114is configured to pulse the backlight 46 only if the camera 44 sensesinsufficient light to otherwise operate properly. As seen in FIG. 5, inthe illustrated embodiment, the lens 76 has a first angled portion 118configured to direct light 98 from the backlight 46 generally onto thesurface 96, and a convex portion 120 for focussing incoming light 100 oncamera sensor 78. In an example embodiment, the lens 76 is slidablymounted within tracks 89 formed on the inside of cover back 72 such thata user can apply force to a small outwardly projecting tab 88 to slidethe lens 76 into the case and out of the line of site of backlight 46and camera sensor 78. A proximity switch or sensor 91 (indicated inphantom on FIG. 4) is connected to the microprocessor 38 to indicate tothe optical interface engine 60 the location of the lens 76. In anexample embodiment, the lens 76 is located in the closed position (asshown in FIG. 5) when the camera is in navigation mode to improve thefocus of the camera and backlight on the near surface 96. In digitalcamera mode, the lens 76 can be opened (slid out of the way) to allowthe camera to focus on further objects. In some embodiments, thebacklight 46 may also be used in camera mode in low light conditions. Inone embodiment, select mode module 110 is configured to toggle betweennavigation mode and camera mode depending on the location of lens 76 asdetected by proximity switch 91.

FIG. 7 shows a block diagram representation of a navigation processcarried out by navigation module 112 according to example embodiments ofthe invention. During the navigation process, the camera 44 periodicallycaptures images of the surface 96 at a predetermined capture rate(typically measured in frames per second). FIG. 7 represents theprocessing of a single image frame. As indicated in step 202, the device10 is configured to capture an image, and as indicated in step 204, theimage is then filtered using methods known in the art to, among otherthings, sharpen contrast and adjust brightness. In navigation mode thecolor capacity and full resolution of the camera 44 is not required,thus to accommodate for the limited processing resources that mobiledevices typically have, in an example embodiment the filtered image isconverted to a sharp contrast black and white or grey scale image, andthen reduced in resolution by, in various embodiments, combining pixelsinto clusters and/or discarding selected pixels as indicated in step206. By way of non-limiting examples, in various example embodiments,the resolution of the image is reduced to a relatively low resolutionimage such as 32 by 32 pixels or 16 by 16 pixels, although otherresolutions can also be used. Such conversion simplifies processing ofthe images.

As indicated in step 208, the converted image is then stored in anavigation image buffer so that it can be compared with preceding andsuccessive images. As indicted in step 210, the stored converted imageis compared with one or more preceding stored converted images todetermine the relative distance that the device 10 has moved since thepreceding image, and the direction of relative movement. Thisinformation is then translated into relative movement along X and Ycoordinates (dX and dY), as indicated in step 212.

By way of non-limiting example, a modified Reichardt algorithm can beused to process the converted, low resolution images to determine dX anddY. In this algorithm, motion is detected by locating the zero-crossingedges of images and determining their appearance or reappearance insequential images. Common features between two sequential images areidentified to determine the distance between them. This information isthen translated into X and Y coordinates. The speed of movement is alsocalculated based on the image capture rate (which is a known value) andthe calculated distance moved between images.

As indicated in Step 214, the on-screen pointer 84 is moved based on dXand dY. In one embodiment, the direction that the on-screen pointer 84is moved corresponds to the calculated direction of movement of thedevice 10. In another embodiment, the on-screen pointer 84 is moved inthe opposite direction of the movement of the device 10. The distancethe on-screen pointer 84 is moved is a scaled value of dX and dY, withthe scaling factor depending on the movement distance and speed. Steps202-214 are repeated continuously while the device 10 is in navigationmode. Once the pointer 84 is in a desired position, the user uses one ormore predetermined keys 92 and 90 for selection and control functions.In various embodiments, some of the steps of FIG. 7 are omitted and/orperformed in an order other than as shown.

Thus, it will be appreciated that the present invention allows anon-board camera 44 to be used as an on-screen navigation device. Invarious embodiments, the camera 44 is located in different locationsthan on the back of the device 10, and the device 10 has differentconfigurations other than the example embodiment described above. Forexample, the camera 44 may be located facing outward from the keyboard32, such that a user can navigate by moving the palm of their hand ortheir thumb over the keyboard area. By way of non-limiting example, FIG.8 shows a front view of a handheld device 100 according to anotherembodiment of the invention. Handheld device 100 is similar to device10, however the keyboard 32 of handheld device 100 includes athumb-activated QWERTY keyboard next to which camera 44 is located, andthe main body or case 102 of the handheld device 100 includes first caseportion 104 and second case portion 106 that are pivotally mountedtogether. Second case portion 106 houses display 22, and the first caseportion 104 houses the keyboard 32, which is configured for thumbtyping. In handheld device 100, the lens 76 for camera 44 and backlight46 is provided through opening 76 on the front of the first case portion104. The camera 44 faces the same direction as the keyboard 32 fordetecting relative motion of a user's hand or thumb over the keyboardsurface of the handheld device 100.

In some embodiments, the camera is pivotally mounted to the case of thehandheld device such that it can be rotated to face in a directiondesired by the user for navigation purposes. By way of example, apivotally mounted camera 44 and backlight unit 108 are shown by dashedlines in FIG. 8. The camera 44 and backlight unit 108 may be detachablefrom case 102.

In some embodiments where camera and scanner modes are not required, alow resolution optical sensor may be used in place of camera 44. In someembodiments, the lens 76 may be removable such that it can be replacedwith a lens adapted specifically for the mode that the device 10 isoperating in—for example a different lens could be used for navigationmode than for camera mode.

The above-described embodiments of the present invention are intended tobe examples only. Alterations, modifications and variations may beeffected to the particular embodiments by those skilled in the artwithout departing from the scope of the invention, which is defined bythe claims appended hereto.

1. A mobile device, comprising: a main body having a front face andopposing back face; a processor and associated memory housed within themain body; a display screen mounted in the front face; an optical sensormounted in the back face for capturing successive images and providingimage signals representative of the captured images to the processor;and a navigation module executable by the processor for converting theimage signals into lower resolution image signals and determining, basedon the lower resolution image signals, a relative movement between themain body and a reference surface and moving a pointer on the displayscreen based on the determined relative movement.
 2. The device of claim1 wherein the navigation module compares the successive images capturedby the optical sensor at a predetermined capture rate to determine arelative direction, distance and speed of movement of the main bodyrelative to the reference surface, and moves the pointer on the displayscreen based on the determined relative direction, distance and speed.3. The device of claim 1 wherein the device further includes a cameramodule executable by the processor for displaying the captured images onthe display screen, the camera module and the navigation module beingselectively operable to cause the captured images to be displayed on thedisplay screen in a camera mode of the device and the pointer to bemoved on-screen based on the captured images in a navigation mode. 4.The device of claim 3, further comprising a select mode moduleexecutable by the processor for selecting between the camera mode andthe navigation mode.
 5. The device of claim 4, wherein said select modemodule receives said captured images and determines, based upon saidcaptured images, whether said optical sensor is within a thresholddistance from the reference surface, and wherein said select mode moduleautomatically selects between said navigation mode and said camera modebased upon said determination.
 6. The device of claim 1 wherein theoptical sensor comprises a high resolution camera.
 7. The device ofclaim 6 wherein the high resolution camera comprises a high resolutioncolor-enabled camera configured to provide high resolution color imagesignals.
 8. The device of claim 7, wherein the navigation module isconfigured to convert the high resolution color image signals into lowerresolution grey scale image signals.
 9. The device of claim 1 whereinthe device is a handheld wireless communications device including acommunications sub-system connected to the processor for exchangingsignals with a wireless network and with the processor.
 10. An on-screennavigation method for a mobile computing device having front face with adisplay screen and having a back face, the method comprising: displayingan on-screen pointer on the display screen; capturing successive imagesof a reference surface using an optical sensor mounted in the back face;converting the successive images into lower resolution images; comparingsuccessive lower resolution images to determine a relative movementbetween the optical sensor and the reference surface; and moving theon-screen pointer on the display screen based on the determinedmovement.
 11. A mobile device, comprising: display means for displayinga pointer; camera means for capturing successive images and providingimage signals representative of the captured images; navigation meansfor receiving the image signals and determining, based on the imagesignals, a relative movement between the main body and a referencesurface and moving a pointer on the display means based on thedetermined relative movement; and a casing having a front face and aback face, and wherein the display means is mounted in the front faceand the camera means is mounted in the back face.
 12. The device ofclaim 11 wherein the navigation means includes means for converting theimage signals to lower resolution image signals and determining therelative movement based on the lower resolution image signals.
 13. Thedevice of claim 12 wherein the camera means includes means for capturinghigh resolution color image signals and the means for convertingincludes means for converting the high resolution color image signals tolower resolution grey scale image signals.
 14. The device of claim 11wherein the navigation means includes means for comparing the successiveimages captured by the camera means at a predetermined capture rate todetermine a relative direction, distance and speed of movement of themain body relative to the reference surface, and means for moving thepointer on the display screen based on the determined relativedirection, distance and speed.
 15. The device of claim 11 wherein thedevice further includes an imaging means executable by the processor fordisplaying the captured images on the display means, the imaging meansand the navigation means being selectively operable to cause thecaptured images to be displayed on the display means in a camera mode ofthe device and the pointer to be moved on-screen based on the capturedimages in a navigation mode.
 16. The device of claim 15, furthercomprising a select mode means for selecting between the camera mode andthe navigation mode.
 17. The device of claim 16, wherein said selectmode means receives said captured images and determines, based upon saidcaptured images, whether said optical sensor is within a thresholddistance from the reference surface, and wherein said select mode meansincludes means for automatically selecting between said navigation modeand said camera mode based upon said determination.